On the reflectance spectroscopy of snow Kokhanovsky, Alexander; Lamare, Maxim; Di Mauro, Biagio ...
The cryosphere,
07/2018, Letnik:
12, Številka:
7
Journal Article
Odprti dostop
We propose a system of analytical equations to retrieve snow grain
size and absorption coefficient of pollutants from snow reflectance or snow
albedo measurements in the visible and near-infrared ...regions of the
electromagnetic spectrum, where snow single-scattering albedo is close to
1.0. It is assumed that ice grains and impurities (e.g., dust, black and
brown carbon) are externally mixed, and that the snow layer is semi-infinite and
vertically and horizontally homogeneous. The influence of close-packing
effects on reflected light intensity are assumed to be small and ignored. The
system of nonlinear equations is solved analytically under the assumption that
impurities have the spectral absorption coefficient, which obey the
Ångström power law, and the impurities influence the registered spectra
only in the visible and not in the near infrared (and vice versa for ice grains).
The theory is validated using spectral reflectance measurements and albedo of
clean and polluted snow at various locations (Antarctica Dome C, European
Alps). A technique to derive the snow albedo (plane and spherical) from
reflectance measurements at a fixed observation geometry is proposed. The
technique also enables the simulation of hyperspectral snow reflectance
measurements in the broad spectral range from ultraviolet to the
near infrared for a given snow surface if the actual
measurements are performed at a restricted number of wavelengths (two to four,
depending on the type of snow and the measurement system).
Cryoconite is rich in natural and artificial
radioactivity, but a discussion about its ability to accumulate
radionuclides is lacking. A characterization of cryoconite from two Alpine
glaciers is ...presented here. Results confirm that cryoconite is significantly
more radioactive than the matrices usually adopted for the environmental
monitoring of radioactivity, such as lichens and mosses, with activity
concentrations exceeding 10 000 Bq kg−1 for single radionuclides. This
makes cryoconite an ideal matrix to investigate the deposition and
occurrence of radioactive species in glacial environments. In addition,
cryoconite can be used to track environmental radioactivity sources. We have
exploited atomic and activity ratios of artificial radionuclides to identify
the sources of the anthropogenic radioactivity accumulated in our samples.
The signature of cryoconite from different Alpine glaciers is compatible
with the stratospheric global fallout and Chernobyl accident products.
Differences are found when considering other geographic contexts. A
comparison with data from literature shows that Alpine cryoconite is
strongly influenced by the Chernobyl fallout, while cryoconite from other
regions is more impacted by events such as nuclear test explosions and
satellite reentries. To explain the accumulation of radionuclides in
cryoconite, the glacial environment as a whole must be considered, and
particularly the interaction between ice, meltwater, cryoconite and
atmospheric deposition. We hypothesize that the impurities originally
preserved into ice and mobilized with meltwater during summer, including
radionuclides, are accumulated in cryoconite because of their affinity for
organic matter, which is abundant in cryoconite. In relation to these
processes, we have explored the possibility of exploiting radioactivity to date
cryoconite.
This study describes the development of a small hyperspectral Unmanned Aircraft System (HyUAS) for measuring Visible and Near-Infrared (VNIR) surface reflectance and sun-induced fluorescence, ...co-registered with high-resolution RGB imagery, to support field spectroscopy surveys and calibration and validation of remote sensing products. The system, namely HyUAS, is based on a multirotor platform equipped with a cost-effective payload composed of a VNIR non-imaging spectrometer and an RGB camera. The spectrometer is connected to a custom entrance optics receptor developed to tune the instrument field-of-view and to obtain systematic measurements of instrument dark-current. The geometric, radiometric and spectral characteristics of the instruments were characterized and calibrated through dedicated laboratory tests. The overall accuracy of HyUAS data was evaluated during a flight campaign in which surface reflectance was compared with ground-based reference measurements. HyUAS data were used to estimate spectral indices and far-red fluorescence for different land covers. RGB images were processed as a high-resolution 3D surface model using structure from motion algorithms. The spectral measurements were accurately geo-located and projected on the digital surface model. The overall results show that: (i) rigorous calibration enabled radiance and reflectance spectra from HyUAS with RRMSE < 10% compared with ground measurements; (ii) the low-flying UAS setup allows retrieving fluorescence in absolute units; (iii) the accurate geo-location of spectra on the digital surface model greatly improves the overall interpretation of reflectance and fluorescence data. In general, the HyUAS was demonstrated to be a reliable system for supporting high-resolution field spectroscopy surveys allowing one to collect systematic measurements at very detailed spatial resolution with a valuable potential for vegetation monitoring studies. Furthermore, it can be considered a useful tool for collecting spatially-distributed observations of reflectance and fluorescence that can be further used for calibration and validation activities of airborne and satellite optical images in the context of the upcoming FLEX mission and the VNIR spectral bands of optical Earth observation missions (i.e., Landsat, Sentinel-2 and Sentinel-3).
Drylands, one of the planet’s largest terrestrial biomes, are suggested to be greatly threatened by climate change. Drylands are usually sparsely vegetated, and biological soil crusts (biocrusts) – ...that is, soil surface communities of cyanobacteria, mosses and/or lichens – can cover up to 70% of dryland cover. As they control key ecosystem processes, monitoring their spatial and temporal distribution can provide highly valuable information. In this study, we examine the potential of European Space Agency’s (ESA) Sentinel-2 (S2) data to characterize the spatial and temporal development of biocrust and vascular plant greening along a rainfall gradient of the Negev Desert (Israel). First, the chlorophyll a absorption feature in the red region (CRred) was identified as the index mostly sensitive to changes in biocrust greening but minimally affected by changes in soil moisture. This index was then computed on the S2 images and enabled monitoring the phenological dynamics of different dryland vegetation components from August 2015 to August 2017. The analysis of multi-temporal S2 images allowed us to successfully track the biocrust greening within 15 days from the first seasonal rain events in the north of Negev, and to identify the maximum development of annual vascular plants and greening of perennial ones. These results show potential for monitoring arid and semi-arid environments using the newly available S2 images, allowing new insights into dryland vegetation dynamics.
Abstract Cryoconite holes are structures characteristic of the glacierised ablation areas, formed from dark sediment deposition which melt the glacier ice leading to their cylindrical structure ...formation. The present study focuses on an unstudied blue ice area of East Antarctica over the Hells Gate Ice Shelf in Northern Victoria Land. Taking advantage of neural network methodology applied on drone acquisitions, an automatic detection of cryoconite holes was carried out (88% of accuracy), mapping ~3500 m 2 (0.4% fractional cover) of cryoconite holes over an area of 900 000 m 2 . The surveyed area showed heterogeneity in cryoconite hole distribution, with some regions showing an area fraction of holes (regions exhibiting frozen ponds and ice bands) as high as 8%. Thanks to this detection, the spatially integrated radiative forcing of cryoconite holes over blue ice surface was estimated to be almost 1 Wm −2 . This estimation was based on the measured albedo difference between the two surfaces, which averages 0.31. However, the radiative forcing is strongly increased if calculated at a single cryoconite hole scale, showing an average of ~200 Wm −2 . In conclusion, the present research represents a first attempt of analysing cryoconite holes as factors impacting the blue ice in Northern Victoria Land, deepening our understanding of their glaciological role at the margins of the East Antarctic Ice Sheet.
Glaciers in many parts of the world are reported to be undergoing a darkening process (decrease in surface albedo). This has multiple causes, including the presence of impurities (such as mineral ...dust, carbonaceous particles, algae and cryoconite), and debris. Also glacier melt can induce an albedo decrease by initiating a melt-albedo feedback cycle. The extent of this albedo decrease and its spatial variability in different mountain ranges however are poorly known. In this study, we assess albedo changes on a selection of Alpine glaciers (542 glaciers >0.1 km2) by deriving eight albedo metrics from MODIS MOD10A1 satellite observations over a time period of 20 years (2000–2019). Albedo metrics include the start and end of the ablation season and its length, calculated through the selection of a threshold (albedo = 0.4) to separate melting ice from snow. We further calculated the number of ice melt days, integral of the length of the ablation season, minimum annual albedo, mean summer albedo and mean annual albedo. We identified significant trends in all metrics for a large percentage of glacier cells. A negative trend in minimum albedo (−0.06 decade−1 on average) was identified for 56% of glacier cells, and 70% of glaciers; a positive trend in the length of season (+18 days decade−1 on average) was identified for 11% of glacier cells and 19% of glaciers. Changes in minimum albedo occur mostly around the glacier median elevation, while for other metrics (e.g. start and length of season) changes are more frequent at the glacier termini (i.e. below 25th elevation percentile). We further investigated the relationship between the metrics and climate (downscaled ERA5 reanalysis data) and topographic (ALOS AW3D30 DEM) variability; we found that average summer temperature and elevation are the most significant proxies for all metrics (maximum correlation = 0.40 between elevation and minimum albedo) compared to spring temperature and other topographic variables. Finally, we looked at the ability of the albedo metrics to represent annual and summertime glacier mass balance of 31 selected Alpine glaciers in the World Glacier Monitoring Service database, finding significant correlations between minimum and summertime albedo and annual mass balance (R2 = 0.44 for minimum albedo and 0.40 for summertime albedo using all observations). Considering individual glaciers, all showed significant correlations with minimum albedo, and some of them are strongly correlated (e.g. Argentiere: R2 = 0.84, Saint Sorlin R2 = 0.88). Our glacier phenology approach opens new perspectives for the remote sensing of alpine and polar glaciers.
•The concept of glacier phenology is here introduced and applied to the European Alps.•Albedo-derived metrics were extracted from 20 years of MODIS satellite data.•56% of glacier cells and 70% of glaciers show negative minimum albedo trends.•Summer glacier albedo significantly correlates with the mass balance of all glaciers.•Ablation season length increased (19% of glaciers) by 18 days/decade on average.
Cryoconite is a mixture of mineral and organic material covering glacial ice, playing important roles in biogeochemical cycles and lowering the albedo of a glacier surface. Understanding the ...differences in structure of cryoconite across the globe can be important in recognizing past and future changes in supraglacial environments and ice-organisms-minerals interactions. Despite the worldwide distribution and over a century of studies, the basic characteristics of cryoconite, including its forms and geochemistry, remain poorly studied. The major purpose of our study is the presentation and description of morphological diversity, chemical and photoautotrophs composition, and organic matter content of cryoconite sampled from 33 polar and mountain glaciers around the globe. Observations revealed that cryoconite is represented by various morphologies including loose and granular forms. Granular cryoconite includes smooth, rounded, or irregularly shaped forms; with some having their surfaces covered by cyanobacteria filaments. The occurrence of granules increased with the organic matter content in cryoconite. Moreover, a major driver of cryoconite colouring was the concentration of organic matter and its interplay with minerals. The structure of cyanobacteria and algae communities in cryoconite differs between glaciers, but representatives of cyanobacteria families Pseudanabaenaceae and Phormidiaceae, and algae families Mesotaeniaceae and Ulotrichaceae were the most common. The most of detected cyanobacterial taxa are known to produce polymeric substances (EPS) that may cement granules. Organic matter content in cryoconite varied between glaciers, ranging from 1% to 38%. The geochemistry of all the investigated samples reflected local sediment sources, except of highly concentrated Pb and Hg in cryoconite collected from European glaciers near industrialized regions, corroborating cryoconite as element-specific collector and potential environmental indicator of anthropogenic activity. Our work supports a notion that cryoconite may be more than just simple sediment and instead exhibits complex structure with relevance for biodiversity and the functioning of glacial ecosystems.
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•The morphology of cryoconite varies greatly between regions.•Cryoconite consists of loose mineral material or various types of granules.•Colour of cryoconite depends on organic matter content and its interplay with minerals.•Cryoconite is a complex structure providing various ecological niches for glacial microbes.
In this paper, we made use of PRISMA imaging spectroscopy data for retrieving surface snow properties in the Nansen Ice Shelf (East Antarctica). PRISMA satellite mission has been launched in 2019 and ...it features 239 spectral bands covering the 400-2500 nm interval. These data are promising for cryospheric applications, since several snow and ice parameters can be derived from reflectance in the Visible Near InfraRed - Short Wave InfraRed (VNIR-SWIR) wavelength interval. Here we analyze, for the first time, PRISMA data collected in Antarctica. Our scene was acquired on December 2020 over the Nansen Ice Shelf (NIS). Using PRISMA data we estimated various snow parameters (effective grain diameter, snow specific surface area, snow spectral and broadband albedo, bottom of atmosphere snow reflectance, type of impurities in snow and their concentration), and we compared them with data presented in the scientific literature.
The input of mineral dust from arid regions impacts snow
optical properties. The induced albedo reduction generally alters the
melting dynamics of the snowpack, resulting in earlier snowmelt. In this
...paper, we evaluate the impact of dust depositions on the melting dynamics of
snowpack at a high-elevation site (2160 m) in the European Alps (Torgnon,
Aosta Valley, Italy) during three hydrological years (2013–2016). These
years were characterized by several Saharan dust events that deposited
significant amounts of mineral dust in the European Alps. We quantify the
shortening of the snow season due to dust deposition by comparing observed snow
depths and those simulated with the Crocus model accounting, or not, for the
impact of impurities. The model was run and tested using meteorological data
from an automated weather station. We propose the use of repeated digital
images for tracking dust deposition and resurfacing in the snowpack. The
good agreement between model prediction and digital images allowed us to
propose the use of an RGB index (i.e. snow darkening index – SDI) for
monitoring dust on snow using images from a digital camera. We also present
a geochemical characterization of dust reaching the Alpine chain during
spring in 2014. Elements found in dust were classified as a function of
their origin and compared with Saharan sources. A strong enrichment in Fe
was observed in snow containing Saharan dust. In our case study, the
comparison between modelling results and observations showed that impurities
deposited in snow anticipated the disappearance of snow up to 38 d a out of
a total 7 months of typical snow duration. This happened for the season
2015–2016 that was characterized by a strong dust deposition event. During
the other seasons considered here (2013–2014 and 2014–2015), the snow
melt-out date was 18 and 11 d earlier, respectively. We conclude that the
effect of the Saharan dust is expected to reduce snow cover duration through
the snow-albedo feedback. This process is known to have a series of further
hydrological and phenological feedback effects that should be characterized
in future research.
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